The Biological Indices of
Stream Quality
Willis M. Van Horn
Research Associate, The Institute of Paper Chemistry
Appleton, Wisconsin
A natural stream or aquatic environment is the result of a com¬
bination of factors, all of which play their own peculiar part in
determining the nature of that environment. These factors may be
physical—such as the stream gradient, barriers such as dams, the nature
of the stream bed, the character of the drainage basin. They may be
chemical in nature—such as the dissolved oxygen content of the water,
the hydrogen-ion concentration, the alkalinity or acidity content, the
amount of solids in suspension or solution, the biochemical oxygen
demand. Or the factors may be biological in nature, in that each
stream normally supports its own characteristic animal and plant asso¬
ciation, existing under the law of natural selection.
The aquatic biologist, in studying and observing any practicular
stream environment, applies certain types of measurement in order
to get adequate information about the conditions which obtain therein.
Thus, by chemical analysis he can determine the oxygen content of
the water, the pH, the alkalinity or acidity, the amount of solids,
and other information. At the same time he may make a population
study, carefully determining the species present and, as nearly as possi¬
ble, the number of individuals of each species. When all the informa¬
tion is available, he is able, by correlating his data, to describe the
environment, relating its inhabitants to their physical and chemical
requirements.
When such studies have been made, it is usually found that any
particular animal and plant association exists under close natural con¬
trol—one might say, equilibrium. Through the operation of the law
of survival, each species exists in the maximum numbers permitted by
the environmental conditions—when individuals in excess of this maxi¬
mum are produced, nature, in one way or another, destroys the excess,
provided the conditions of the environment remain constant. If, how¬
ever, the environmental conditions change, as in the case of pollution,
and this change affords an advantage to one or two species that the
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